As package processing techniques for foods such as fresh meat, ham, sausages, cheese and other products, techniques in which a heat-shrinkable multilayer film is formed into a bag or pouch by a bag-forming machine and then filled with contents, or the contents are submitted to automatic packaging, either directly or loaded on a tray, during bag-forming processing have been conventionally used. Thus, the characteristics required of such a film for packaging are diverse, and an excellent balance of various characteristics such as heat-shrinkability, strength, heat resistance, moldability, suitability for various types of packaging, gas barrier capability, moisture barrier capability, and the like is required. As films suitable for packaging material applications that require strength above all, heat-shrinkable multilayer films containing a polyamide-based resin layer as a primary resin layer have been proposed.
Furthermore, when a meat product of indeterminate form, such as fresh fish or fresh meat or processed meat such as roast pork, beef or chicken, is packaged by being put in a packaging material such as a bag or pouch comprising a multilayer film, it is packaged by vacuum-packaging the meat product and then shrinking the packaging material using hot water of approximately 75 to 95° C., for example. Thus, it is preferable for the packaging material to exhibit good heat-shrinkability and to provide a good appearance by preventing seepage and retention of juice in the excess portion (normally called the “edge”) between the contents and the seal portion used for forming a bag or pouch after packaging. To do so, it is desirable for the multilayer film to be suitable for fusing together of the inner surface resin layers of the edge of the bag or pouch filled with contents (called “self-welding” hereinafter) by heating during heat-shrinking. In other words, it is desirable for the inner surface layer of the multilayer film to exhibit “self-weldability.” In particular, in the course of transport, storage, and the like after production of the packaged meat product described above, the self-welded inner surface layers of the edge peel due to shock such as collision with other articles or falling, and a pool of juice forms in the edge, appearance becomes poor, and a tight fit is not retained. Therefore, in the case of a packaged meat product that was packaged a long distance from the consumer or that will be consumed long after it was packaged, it is preferable for the adhesion strength of the inner surface layer by heat-shrinking (that is, self-weld strength) to be sufficiently high and close to the adhesion strength of the seal portion formed during packaging (that is, seal strength). Needless to say, like conventional food packaging films, such packaging films of meat products also require an excellent balance of various characteristics such as heat-shrinkability, strength, heat resistance, moldability, suitability for various types of packaging, gas barrier capability, and moisture barrier capability, as well as suitability for rapid sealing to enable rapid bag-forming and content filling. Additionally, packaging films for meat products such as fresh or processed meat also require transparency to enable visibility of the contents, gloss to provide a good appearance, and suitability for printing or label adhesion for displaying administrative information such as the location of production, producer, processing date, and processor.
However, the actual situation is that a multilayer film material that satisfies to a high degree the diversity of characteristics described above required in packaging films for meat products such as fresh and processed meat has not been obtained. For example, Patent Documents 1 to 3 disclose heat-shrinkable multilayer films having self-weldability, but when the above-described harsh transport or storage conditions incurred by packaged meat products are taken into consideration, the self-weldability thereof is not satisfactory, and their strength, surface gloss, suitability for printing, label adhesion, and rapid sealing are also insufficient.